Repair and maintenance of the civil infrastructure, including roads and highways of the United States presents great technical and financial challenges. The American Association of State Highway Transportation Officials (AASHTO) issued a bottom line report in 2010 stating that $160 billion a year must be spent to maintain infrastructure; however, only about $80 billion is being spent. The result is a rapidly failing infrastructure. New methods of maintaining existing roads and new methods of constructing roads that would extend the useful life for the same budget dollar are needed to meet the challenges of addressing our failing infrastructure.
In the United States alone there are approximately 4.4 million center lane miles of asphalt concrete, with a center lane comprising a 24 foot wide pavement surface having a lane in each direction. Asphalt concrete paving surfaces are typically prepared by heating aggregate to 400° F., and applying liquid asphalt (e.g., by spraying into a pug mill or drum coating) to yield a mixture of 95% aggregate and 5% asphalt. If a temperature of approximately 350° F. is maintained for the mixture, it is considered hot mix asphalt and does not stick to itself as long as the temperature is maintained. The hot mix asphalt is typically placed in a transfer truck, which hauls it to the job site, where it is placed on either a gravel road base or onto an old road surface that has been previously primed. A paving apparatus receives the hot mix asphalt from the transfer truck and spreads it out uniformly across the base surface, and as the material progressively cools below 250° F. degrees it is compacted with a roller. The hot mix asphalt is rolled to a uniform density, and after approximately one to three days of cooling and aging the surface can be opened to traffic.
After such asphalt pavement has been in place for several years, the pavement progressively ages. Water works its way into the pavement. It begins to lose its integrity on the surface, causing aggregate at the surface of the pavement to be lost. The pavement surface roughens as aggregate is lost, and cracks begin to form. Conventional pavement repair techniques at this stage in the deterioration process include: pouring hot rubber asphalt into the cracks, using cold patch (a polymer-modified cold mix asphalt that can be applied to a damaged road surface, e.g., placed in a pothole, under ambient temperature conditions using hand tools). Another technique for repairing pavement exhibiting minimal damage involves application of a liquid asphalt emulsion to the pavement surface so as to provide a degree of waterproofing to slow the aging process, or, for surfaces exhibiting more deterioration, application of a thin layer of a slurry of aggregate and asphalt emulsion over the top of the pavement.
Preparing and installing hot asphalt pavement involves running aggregate through a heat tube (typically at around 400° F.) where moisture is driven off to prevent boil over when the rock contacts molten asphalt. The aggregate is added to asphalt, optionally containing a rubber polymer. The aggregate is sent through a mill having high velocity tines that rolls the aggregate through a spray of asphalt. The resulting mixture of aggregate with baked-on asphalt typically comprises 95% aggregate and 5% asphalt (optionally with rubber polymer). The mixture exits the mill at about 350° F. and is transported into waiting trucks (e.g., a belly dump truck) which are driven to the job site. New pavement is laid down over an earthen base covered with gravel that has been graded and compacted. Typically, the new road is not laid in a single pass. Instead, a first 2-3 inch lift of loose hot asphalt is laid down and partially compacted, and then a second lift is laid over the first and compacted. The temperature of the asphalt concrete pavement at this stage is typically about 140° F. Additional lifts can be added as desired, e.g., to a depth of approximately 12 inches, depending upon the expected usage conditions for the road (heavy or light transportation, the velocity of traffic, desired lifetime). Primer or additional material is typically not put between layers of lift in new construction, as the fresh pavement exhibits good adherence to itself in new construction. New construction design typically never requires any primer or additional material between the subsequent lifts.
After approximately fifteen years of exposure to the elements, it becomes cost prohibitive to attempt to maintain asphalt pavement via conventional cold patching, waterproofing, and slurry techniques. The conventional approach at this stage in the deterioration of the pavement typically involves priming the damages surface and applying a layer of hot mix asphalt. For pavement too deteriorated for application priming and application of a layer of hot mix asphalt, a cold-in-place recycling process can be employed. In cold-in-place recycling, typically the topmost 2 to 5 inches of the damaged road surface are pulverized down to a specific aggregate size and mixed with an asphalt emulsion, and then re-installed to pave the same road from which the old paving material has been removed.
Existing pavement (asphalt or concrete) is typically repaired by use of an overlay, e.g., a mixture of aggregate and asphalt such as described above for new road construction. In the case of repaving over the top of rigid concrete, some type of primer is typically applied, e.g., as a spray resulting in application of approximately 10 gallons of primer per 1,000 square feet of pavement. The primer can be an asphalt emulsion that provides a tacky surface for the new overlay. A single layer of overlay can be applied, or multiple layers, typically two or more.
Cracks and stresses in a repaired underlying road bed will quickly imprint themselves on new overlays of paving material, due to the malleability of the new asphalt under rolling loads. As the underlying road bed undergoes expansion and contraction under ambient condition, cracks can be telegraphed up through as much as three inches of overlying asphalt. A conventional method for achieving some resistance to the telegraphing of old defects in the underlying road bed is to put down a hot tack coat of asphalt, lay a polypropylene mat (similar in appearance to spun-bond polypropylene, typically ¼-½ inches in thickness, available as Petromat® from Nilex, Inc. of Centennial, Colo.) over the hot tack coat of asphalt, followed by a layer of new hot asphalt concrete which is then compacted over the existing surface. This will inhibit the rate of telegraphing of cracks to a limited extent, such that instead of taking place from 6 months to 2 years after repair, the cracks do not telegraph for from to 1 year to 3 years after repair. This telegraphing phenomenon by the defects in an existing aged roadbed manifest surface defects in a new pavement overlay about three times sooner than is common to a fresh asphalt concrete pavement placed on a compacted earthen and gravel base; as is the practice in new construction.
Deterioration mechanisms of new highways have been investigated over a 20 year life cycle. Overlays are typically applied between the twelfth and fifteenth year. Typically, no significant deterioration is observed over the first five years of a well-built highway. Within the first five years, cracks or potholes typically do not appear unless there is acute damage to the pavement, or loose material underneath the pavement. After the first five years, physical symptoms of deterioration are observed, including lateral and longitudinal cracks due to shrinkage of the pavement mass through the loss of binder and embrittlement of the asphalt. Cracks ultimately result in creation of a pothole. Ravelling is a mechanism wherein the effects of exposure to water and sun break down the adhesion between the rock on the top surface of the pavement and the underlying aggregate, such that small and then larger rock is released from the pavement. A stress fracture is where the pavement, for one reason or another, may not have been thick enough to withstand exposure to an extremely heavy load, moisture, or poor compaction underneath. When combined with shrinkage of the asphalt itself as it goes through heating and cooling cycles, and application of oxidative stress, stress fractures can also result. Stress fractures are characterized by extending in different directions (unlike the lateral or longitudinal cracking as described above).
The macro-texture of a pavement refers to the visible roughness of the pavement surface as a whole. The primary function of the macro-texture is to help maintain adequate skid resistance to vehicles travelling at high speeds. It also provides paths for water to escape which helps to prevent wheels of motor vehicles from hydroplaning. This optionally may be accomplished through cutting or forming grooves in existing or new pavements. Micro-textures refer to the roughness of the surface of the individual stones within the asphalt concrete pavement. It is the fine texture that occurs on chippings and other exposed parts of the surfacing. For concrete pavement this is usually the sand and fine aggregates present at the surface layer and for asphalt it is usually associated with the type of aggregates used. Micro-texture creates frictional properties for vehicles travelling at low speeds. The wet skid resistant nature of a road is dependent on the interaction of the tire and the combined macro-texture and micro-texture of the road surface.
Conventional repair of shallow surface fissures and ravelling uses various methods. Re-saturants are materials that soften old asphalt. They are typically mixed with an emulsion and sprayed onto the surface of the old pavement. The material penetrates into the uppermost 20 or 30 mils of the pavement and softens the asphalt, imparting flexibility. Thermally fluidized hot asphalt can also be sprayed directly onto the surface, which hardens and provides waterproofing. A fog seal is typically sprayed on the surface, and can be provided with a sand blotter to improve the friction coefficient. In a chip seal, a rubberized emulsion can also be sprayed onto the aged pavement, and then stone is broadcast into the rubberized emulsion which then hardens, bonding the stone. Slurry seal employs a cold aggregate/asphalt mixture prepared in a pug mill and placed on the aged pavement surface, but is applied in a much thinner layer, e.g., 0.25-0.75 inches. Once the pavement surface is repaired, any safety markings can be repainted.
The Federal Highway Administration, through the National Academy of Sciences, has done research into pavement durability. A 20-year long-term paving program (LTPP) was initiated in 1984 in an attempt to understand the failure mechanisms of paving. At the end of the 20-year program and after five years of data analysis, better ways have been developed for measuring pavement failure, the most noteworthy being the Strategic Highway Research Program (SHRP) grading system. The SHRP system can be used to determine the physical qualities of an asphalt product and its potential for long-term service. Subsequently, mechanical testing was developed to determine when the ductility and flexibility of the pavement was diminished, which correlates with end of its useful life as well as the chemical changes in the asphalt itself over time were studied. The presence of carboxylates and sulfoxides that are generated over the life of the pavement cross-section was discovered to be associated with asphalt embrittlement. This discovery now enables prediction of useful life. Accelerated weathering chambers also can be employed to determine the rate of formation of these telltale carboxylates and sulfoxides in a new binder system, binder/aggregate combination, or other paving material thereby predicting an expected useful life. In terms of the chemistry of deterioration, study data indicate that asphalt pavement fails because it becomes brittle. Embrittlement leads to mass loss, which leads to shrinkage, which produces cracks. Cracks become potholes, the pavement stops flexing, and aggregate becomes dislodged.
Deterioration of asphalt binder is generally associated with asphalt beyond the first 100 microns covering the rock surface. An asphalt layer on aggregate at depths within 100 microns of the asphalt/rock interface was found by the 20 year LTDP study to have not experienced the presence of sulfoxides and carboxylates that are associated with embrittlement. Therefore the properties of that asphalt were similar to those of virgin asphalt initially placed on the rock. While not wishing to be bound by theory, it is believed that the tight bond of the asphalt within the first 100 microns of the rock surface exhibited a high degree of intimacy. This intimacy inhibits the movement of scavenging oxidizers into the asphalt structure, thereby minimizing deterioration. Accordingly, it is believed that in an aged paving material averaging 95% aggregate and 5% asphalt, a 100 micron layer of good asphalt surrounds each aggregate particle, with embrittled asphalt in between. It is this “embrittlement zone” where ductility is lost and failure takes place. Air gaps in the cross-section of the pavement can allow water and air to gain access to the asphalt rock interface. Over a period of time, the asphalt goes from being flexible to becoming brittle. The chemistries associated with the embrittlement are related to the formation of sulfoxide or hydroxyl groups, and typically there is a loss of a hydrogen atom on the carbon (oxidation) which causes the key molecular structures to become shorter, thereby less flexible. Once that happens, the pavement becomes inflexible, cracks open up, the pavement loses mass, and rolling loads break up the pavement, causing cracking, potholes, running, ravelling, and block cracking, each resulting in a loss of the pavement integrity.
The conventional methods for repair of surface defects inclusive of rejuvenators and fog seals typically do not exhibit a desirable lifespan. The most durable conventional repair, a slurry seal or a chip seal, may last only 7 or 8 years. An analysis of pavement failure mechanisms provides an explanation for the poor lifespan observed for new asphalt pavement and subsequent repairs. The primary factor is that the repairs do not remedy the underlying embrittlement of the asphalt binder deep within the pavement cross-section. The embrittlement results from the scissioning of the polymer chains present in the asphalt under the influence of free radicals associated principally with water. Water penetrates the pavement, and sunlight and traffic over the pavement surface provides energy for reaction with oxygen and other pavement components, yielding sulfoxide and carboxylate reaction products and reduced polymer chain length through reaction with the resulting free radicals. Loss of polymeric molecular weight impacts the ability of the pavement to stretch and flex. A secondary failure mechanism is loss of rock itself due to hydrolytic attack of the asphalt-rock interface. Rocks typically comprise metal oxides (e.g., calcium oxide, silicon dioxide, lithium oxide, potassium oxide, sodium oxide). Hydroxide groups can form upon exposure to water, resulting in oxidative reactions that impair the adhesion of asphalt to the rock surface, a process referred to as stripping.
Loss of waterproofing typically is a top down mechanism. The asphalt breaks down from exposure to heavy load and the sun, causing water to penetrate between the asphalt and rock. The asphalt can lose its hydrophobicity, with paraffinic components being broken down into more hydrophilic components, which in turn accelerate the process of water adsorption. Ravelling occurs, resulting in a loss of macrotexture. Ultimately, the microtexture of the surface is lost due to abrasion of tires across the surface rubbing off the asphalt and polishing the rock surface, whereby the coefficient of friction drops to unacceptable levels. Typically, a brand new pavement will have a coefficient of friction of between 0.6 and 0.7. Over time, loss of microtexture and ultimately macrotexture results in the coefficient of friction dropping to below about 0.35, at which point the pavement becomes inherently unsafe in terms of steer resistance in the presence of water. Even if a pavement surface doesn't have ravelling or cracking, it can still be unsafe to drive on due to loss of adequate surface texture. Microtexture and macrotexture mechanisms function at different speeds. Typically, up to about 45 mph the microtexture controls stopping distance. Between 45 and 50 the macrotexture begins to have a greater effect on stopping distance, and above 50 mph the macrotexture is the principal determining factor in stopping distance.
Accordingly, there are a variety of maintenance techniques that can be employed on damaged asphalt pavement, some of them more successful than others in preserving and extending the useful life of the pavement. It is known that for pavement that is timely and properly maintained, and repaired in the early stages of deterioration, the typical useful life can be extended out to 19 or 20 years. However, in the current economic environment, the conventional approach to road maintenance is to fix the most often travelled pavement first, and then repair, as budgets allow, progressively the better pavement, such that a useful life closer to 12 or 13 years is typically observed.